Significance Monogamy and extensive paternal care are rare traits in nonhuman primates andare important for the light they shed on the organization of primate social systems. Objectives The long-range objective of this program is to determine the behavioral and physiological processes that contribute to differences in social organization between the monogamous titi monkey (Callicebus) and the polygamous squirrel monkey (Saimiri). Male titi monkeys are heavily involved in the care of their infants whereas male squirrel monkeys avoid infants. The specific aim of this project is to examine the effects of parenthood on the reactions of male and female titi monkeys to infants. Results Attraction to infants has been measured in retrieval tests. To date, one adult male and six adult females have been tested in a before-after design First, as reproductively mature siblings living in their family groups; later, as parents with their first surviving infant. The effects of becoming a parent were variable, ranging from dramatic increases in the tendency to retrieve in some animals, to no change in others. The male failed to retrieve entirely when tested with its infant sibling, but retrieved on every trial with its first-born infant. At the other extreme, three females failed to retrieve consistently in either phase. Future Directions Based on performance of veteran parents, the number of consistent retrievers is expected to increase with additional parental experience. Hormonal changes accompanying changes in parental behavior will be examined in future research. KEYWORDS monogamy, parenting